1. Field of the Invention
This invention relates to a method and an apparatus for detecting colored foreign particles in quartz powder material, and particularly to a method and an apparatus wherein pale colored foreign particles such as iron-based refuse, organic refuse and carbon-based refuse can be detected in a high precision and removed even if these colored foreign particles are contained in white quartz powder material having polygonal external surfaces and tending to reflect light diffusely.
2. Description of the Related Art
As the conventional method of removing foreign particles contained in powder material is mentioned a method wherein powder material existing on a conveyor made of white light-transmissive material is simultaneously irradiated in an inspection unit with a light transmitted from the lower side of the conveyor and with a light reflected on the upper side thereof so as to detect foreign particles with a line CCD camera located above the conveyor as described, for example, in JP-A-2000-84496.
As the conventional detection means for detecting colored foreign particles contained in white powder material is useful a method that a monochrome image of powder material conveyed, for example, on a conveyor is shot with a differential interference microscope and a spatial filter is applied to the shot monochrome image to emphasize a section of changing luminance and thereafter the emphasized section is binarized to detect the colored foreign particles contained in the white powder material.
According to such a method, however, the colored foreign particles can be effectively detected when a shading difference between white powder and the colored foreign particles is large, while there is a problem that when color of the colored foreign particles is pale like light yellow, pale colored foreign particles contained in white powder material cannot be detected sufficiently.
As a detection means for clarifying the shading difference is useful a method that powder material on a conveyor is irradiated in an inspection unit with a light from the upper side of the conveyor and shot with a line CCD camera located above the conveyor so as to process a shot image as a colored image to thereby detect colored foreign particles.
Since this detecting means for the colored foreign particles detects colored foreign particles as color images, even when color of the colored foreign particles is pale like light yellow, the shading of the colored foreign particles is distinguished from white powder material having no polygonal external surface such as powdery or granular medicine, and it is possible in principle to detect the particles in a high precision.
However, when the white powder material is quartz powder material, there is the following problem. Since the quartz powder material has polygonal, glassy external surfaces and tends to reflect light irradiated on the external surfaces of the quartz powder material diffusely, when the quartz powder material existing on a conveyor is irradiated in an inspection unit with a light from the upper side of the conveyor to shoot an image thereof with a line CCD camera located above the conveyor, a part of the quartz powder material is looked black or the like due to the influence of the above diffuse reflection, and hence the color image with accurate shading cannot be obtained, and especially pale colored foreign particles such as iron-based refuse, organic refuse and carbon-based refuse contained in the quartz powder material cannot be detected in a high precision.